This invention relates generally to transmitters and in particular to transmitters that are used to transmit digital HDTV signals.
Television transmitters generally use vacuum tube amplifiers because of their very large power handling ability. Vacuum tube amplifiers have a non linear operating characteristic, which causes signal distortion, especially at high power levels, which are the most efficient levels for the transmitter operators. For conventional NTSC analog transmissions, the distortion is generally tolerable and it is not unusual to operate such vacuum tube amplifiers at a 60 kilowatt level. In most instances, an attempt is periodically made to minimize the effects of the distortion caused by the power amplifier by sampling the output of the transmitter while transmitting a known training signal and manually performing corrective measures on the transmitter IF amplifier to compensate for distortions introduced into the training signal. This is a trial and error approach which is time consuming and highly dependent upon the skill of the technician. It also must be repeated periodically and is generally done "off-line", i.e. when the transmitter is off the air. This is highly inconvenient for continuous transmitters.
With the imminent advent of HDTV television signal broadcasting, the distortion problem has taken on much greater significance. The newly adopted digital HDTV signal is an 8VSB signal consisting of multilevel symbols. When such a signal is processed by state of the art vacuum tube power amplifier tubes, the operating signal power is on the order of -12 dB from the normal 60 kilowatt level for NTSC signals. This power limitation is necessary because of the impairment suffered by the digital signal in non linear transmitters operating near the 60 kilowatt level, and results in severe operating inefficiencies for the transmitter operator and a significant curtailment of the transmission range of the HDTV signal. The distortion introduced is also non linear in that the gain of the amplifier changes with changes in input signal amplitude. Such distortion may be compensated by non linear precorrection of the input signal to offset the amplifier-introduced distortion.
The present invention is directed to developing an on-line correction signal for a non linear precorrector in the transmitter. The invention permits conventional transmitter power amplifiers to be operated with acceptable distortion at their normal 60 kilowatt levels with an HDTV 8VSB multilevel symbol digital television signal.